NASA investigates sending CubeSats to Phobos and back

NASA's Innovative Advanced Concepts Program provides funding to study a small number of highly advanced spaceflight concepts, with the goal of understanding the technological possibilities which will guide the development of future space missions. Under this program, a JPL (Jet Propulsion Laboratory) researcher has proposed the use of a pair of CubeSats for an autonomous mission to retrieve samples from Phobos, Mars' larger moon.

In its simplest form, a CubeSat is a cubical picosatellite that usually has an edge of 10 cm, a volume of a liter, and a mass less than 1.33 kg (2.9 lb). Several CubeSats can be connected together when needed to form a larger vehicle. They are built to strict specifications, so that CubeSats can hitchhike to space together with larger payloads without interfering with the primary mission, which helps keep the typical cost of a CubeSat mission to around US$100K. While CubeSats are usually released in relatively low Earth orbit, this is not a fundamental limitation.

So how do CubeSats get to Phobos on a budget? The study mission is based on the use of two coupled CubeSats, one of which is specialized as the drive vehicle and the other as the sample collector. A European study of a small, solar-powered ion motor for small satellites such as CubeSats has recently appeared, that suggests its use for lunar missions. The JPL study, however, focuses on solar sails.

Once placed in Earth orbit, the drive vehicle deploys a solar sail, which produces a thrust that can be controlled in magnitude and direction by embedded nanoactuators. This thrust slowly increases the altitude of the coupled CubeSats and directs them toward a suitable Lagrange point - perhaps the Earth-Moon L1 point located between the Earth and the Moon.

The Interplanetary Superhighway (Image: NASA)

The Lagrange points in the solar system are passageways into the gravitationally defined Interplanetary Transport Network. This network is a collection of very low-energy orbits which connect the Lagrange points of the solar system. When the CubeSats are inserted into such a transfer orbit, virtually no energy input is required to travel to a similar Lagrange point near Mars. The energy difference between the CubeSats in Earth orbit and the CubeSats in Mars orbit when transferred via the Transport Network is supplied by other planetary bodies through gravitational slingshot maneuvers, so requires no energy input. True, the transfer orbit will be lengthy and indirect, and typically requires far longer than would a traditional Hohmann transfer orbit. However, the Hohmann transfer orbit to Mars orbit would require a change in velocity of about 6 km/s (3.7 miles/s) - a very expensive requirement.

Once the coupled CubeSats are in the vicinity of Mars, how do they manage to collect a sample of Phobos' surface material? Landing on Phobos is not a difficult feat, as the escape velocity is just over 10 meters per second (22 mph). However, the solar sails do not provide the force required to lift the lander CubeSat from Phobos, so it will have to be pulled free by the momentum of the drive CubeSat.

In this scenario, the two CubeSats will skim the surface of Phobos from a hyperbolic orbit. As the lander CubeSat approaches the surface, its motion relative to the surface is stopped by the action of a preset spring which pushes apart the two CubeSats. It is on the surface with very little relative motion, so it has "landed." A robotic scoop might be used to collect a surface sample, but even a sticky surface could do so.

If the two CubeSats remain connected by a tether, when the tether pulls tight both CubeSats will pull free of Phobos' gravitation. The CubeSats then navigate back to the Interplanetary Transit Network, and enter an appropriate zero-energy trajectory to return to Earth. Presumably the CubeSats would eventually dock with the ISS for recovery and transport to the surface.

The mission outlined here is not necessarily the easiest nor the most effective way to sample Phobos using a pair of CubeSats. However, it does illustrate the possibility of such scenarios. The real payoff of the study is to demonstrate the enormous potential of non-brute force approaches to space exploration - there are other ways to travel to other planets than by launching a nuclear-powered spacecraft.

From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer. All articles by Brian Dodson

The problem with using the bungy cord method to tether the cube sats is that Newtons laws of motion still apply. In this case on the other cube sat, like a bat and ball. The casing may be built like a brick, but the Solar Cells, Aerials and Ion Engines/Thrusters with their delicate palladium gauze are not.

If it was an Orion sized Spacecraft with the cube sat at the end of the tether, it could be reeled back in and the energy stored in flywheels and shock absorbers in the payload bay. Which may be the better method, with cube sats docked together they house a 300 metre monofilament carbon fibe cable with a tiny light penetrator with your sticky surface which is fired onto the moons surface, springs out and then reeled back in.

Your choice NASA.

L1ma 4th April, 2012 @ 12:07 am PDT

re; L1ma

The two cubes do not need to reel themselves back together once they have escaped Phobos the sail craft can maneuver to dock with the lander cube. All that is needed to lift the lander is a simple friction brake to prevent shock when the tether runs out of slack.

Slowburn 4th April, 2012 @ 02:39 am PDT

Ode to the idea of an unlimited budget, an unlimited R&D, an unlimited fuel supply and an unlimited amount of space craft.

And a HUGE poo-poo to the DNA frying galactic radiation and the apparent light speed limitation.

Mr Stiffy 4th April, 2012 @ 02:52 am PDT

Re; Slowburn

Friction breaking does work, but I would hope to apply it to the reel, not the line. A larger surface area is available in the reel which would also house both motor and flywheel, with less chance of the break pads breaking off and severing the cable.

Because the escape velocity of Phobos is 11.3 ms it may be even better to drop the paired microsats onto the surface in the form of a lander by aiming ahead of the orbit of Phobos and simply drifting onto it, using one of them to fire the other half back to earth.

L1ma 4th April, 2012 @ 09:49 am PDT

re; Slowburn

Still there is the problem of trying to even out forces in 3 dimensions on 2 objects now acting upon each other via a cord at several hundred meters above a large rotating body 30 minutes transmission away from your control room.

L1ma 4th April, 2012 @ 09:54 am PDT

re; L1ma

Sense they are in orbit around Mars with only the slightest bit of encouragement the two masses at the ends of the cord will assume an orbit at the center of the combined mass. One will be high the other will be low by severing the cord at the lander cube it will enter a predictable orbit and then sever the cord from the sail cube reducing the mass. If you used a reel get rid of it as well. Then as I said before fly the sail cube into docking.

Slowburn 5th April, 2012 @ 01:15 am PDT

It would be nice if this article gave a timescale for this mission. Are we talking years or decades to travel this energy free superhighway?

DR.ZARKOF 5th April, 2012 @ 02:38 pm PDT

re: Dr.Zarkof

The usual gravity assisted path usually takes years off a journey, borrowing energy from a planet and giving speed to the Micro sat (The Voyagers and Gallaleo missions). The cubesat pictured has solar sails, sending it close to the sun would give it a vast initial speed boost especially if Mars is in opposition to the Earth both from gravity assistence and the solar wind, but you are right the point of the solar sails is free thrust through the path of least gravital resistance through planetary Lagrange points which as you correctly surmise will be years longer than necessary.

In other words a mission which will take years to arrive, using space tethers which have the highest rate of failure of any space technology to the planet which also has the highest mission fail rate.

L1ma 6th April, 2012 @ 08:08 am PDT

re; L1ma

Given the energy budget minimum power required path is the minimum time path.

This is why you don't want to use a reel to bring the two modules back together.

Slowburn 6th April, 2012 @ 09:43 pm PDT

re; Slowburn

Not quite true, Most efficent and quickest are not the same thing, if you send your probe through the most energy efficient route I believe it would take 2 - 4 years, the quickest journey time to Mars is 6 months or less at near closest approach. This is with nuclear powered rockets, the quickest route is always dependant upon borrowing speed from a planets mass as well as having the most powerful propulsion, when Mars is at opposition the plan is to use a Venus slingshot orbit to gain velocity.

If you use the slow route, and the Chinese get to know about it, the China mars orbiter return sample probe will use the fast option. Chang'e 2 has already completed its moon mission and has been moved to the Earths L2 point, the next version only needs larger fuel tanks(Chang'e 3 in 2013 is meant to have a sample return rover). However you get there it takes the same amount of energy to get to the Moon as to Mars. But its pointless to be the first to get there and the last to return, the only credo to using Micro sats is that you send a cloud of them at the same time with the knowledge that the high failure rate of Mars missions means at least one will return. So far we have no sucessful use of space tethers or solar sails either.

L1ma 7th April, 2012 @ 03:27 am PDT

As a futher comment. The solar sail at Mars orbit has roughly half the light and solar wind intensity as at Earth orbit. However acting as a solar concentrator it wil give a 5*5cm solar cell tfhe entire energy of a 1 metre square sail, meaning you can switch from solar to ion drive or more likely use ion drive from the outset with a fraction of the mass of a full solar panel.

L1ma 8th April, 2012 @ 07:09 am PDT

re; L1ma

I may not have expressed myself well.

The change of velocity generated by solar sails is extremely slight per second. Therefor a high energy flight will take far longer than the minimum energy path.

Slowburn 8th April, 2012 @ 11:06 am PDT

re; L1ma

The ion rocket uses reaction mass, which gives a finite maximum change in velocity something the solar sail is not limited by.

Slowburn 9th April, 2012 @ 08:28 am PDT

Re; Slowburn

"The change of velocity generated by solar sails is extremely slight per second. Therefore a high energy flight will take far longer than the minimum energy path."

The closer to the Sun you are the more energy per m'2 is available, your Solar sail at close to the Suns surface when unfurled could launch your Micro sat out of the Solar system at near relativistic velocities. It is one of the methods chosen to launch interstellar probes.

The solar sail here is the main form of propulsion, however you need to 'tack' to go in a direction, which massively reduces speed. Because we have launched to the outer solar system we cannot sail this ship home, the return trip would take decades, the sail would be used as a break. Getting out of Mars gravity well with Sail power would be an epic in itself, the probe would then slowly drop into the Earths gravity.

However it is excellent if you combine it with Ion drives If you use the sail as a solar concentrator to provide extra power to solar cells; that extra power can be used in the Ion drive propusion. If Mars is in opposition, you cannot sail towards the Sun, but the Ion drive can power you to Mercurys orbit where you switch to solar sails only until you get to Mars performing the mission and back on the intial leg of the return trip.

At Mars orbit using Ion drive allows the probe to decelerate and manover to Phobos orbit , using the Soalr sails as a solar concentrator only saves a couple of Kilo's of Solar cell weight but this is a lot of reaction mass for an Ion drive.

It may be better to fire off that second Microsat to be a pernament science station on the surface, there is no need for it to return at all.

Its very 19th century steam ship, but they were still quicker than sail alone and when the wind was against them used their engines to power home.

L1ma 10th April, 2012 @ 04:35 am PDT

re; L1ma

First you are adding the mass and volume of the ion engine to the probe. This would probably require a third cube. I am not arguing the utility of the ion engine I just don't see it being worth the cost on this mission.

On the way Mars an inner planet gravity maneuver could conceivably make sense but I do not see the need.

Given the size limitations I do not see leaving the probe on Phobos as a permanent science station as being useful. Which is entirely different than thinking that dumping mass is not a good idea.

19th century steam ships were not notably faster than sail ships (particularly clipper ships) just more reliable in keeping to schedules.

Slowburn 10th April, 2012 @ 02:43 pm PDT

re; L1ma

First you are adding the mass and volume of the ion engine to the probe. This would probably require a third cube. I am not arguing the utility of the ion engine I just don't see it being worth the cost on this mission.

On the way Mars an inner planet gravity maneuver could conceivably make sense but I do not see the need.

Given the size limitations I do not see leaving the probe on Phobos as a permanent science station as being useful. Which is entirely different than thinking that dumping mass is not a good idea.

19th century steam ships were not notably faster than sail ships (particularly clipper ships) just more reliable in keeping to schedules.

Slowburn 10th April, 2012 @ 08:13 pm PDT

Re; Slowburn

"First you are adding the mass and volume of the ion engine to the probe. This would probably require a third cube. I am not arguing the utility of the ion engine I just don't see it being worth the cost on this mission."

Ion engines can be tiny tiny - they are used as correctional thrusters on normal Satelites, this mission only needs one. In fact there is no real mass penalty. They do not rely on power but duration of burn.

The Nstar thruster was nearly 17 kg with 85kg of xenon fuel, launched in 1998 its mission encounterd Asteroied Braille in 1999 then comet Borrelly in 2001. For the same type of mission the fuel would be 9kg of Mecury. This sat is ultra tiny, it would need less than a 1/10th scale thruster and because it has sails we are not worried about it running out of fuel.

It is now 2012, we can grow these things on graphine if we want.

"Given the size limitations I do not see leaving the probe on Phobos as a permanent science station as being useful. Which is entirely different than thinking that dumping mass is not a good idea. "

Remember on a sample return mission you only want the sample back, If thats the case leaving as much mass as possible behind makes sense, including the cable. However we want it left behind and not whipping around hitting our sail. So severing it while leaving it attached to phobos with the surplus mass is a good idea. The second Micro sat on Phobos would have contained the fuel which got us here, now empty, it could also house the reel, we dont want this thing back so all it has is a penetrator and a sample tray which is fired back up the cable to the return Micro sat.

L1ma 11th April, 2012 @ 04:04 am PDT

re; L1ma

With a properly calibrated friction brake the recoil will be minimal and being under sail or thrust should keep it stung out until you have achieved an orbit that does so. while I would prefer a straight stiff tether I can not think of any that would not have accept the coiled shape by the time it reached Mars orbit.

It is not that the ion drive system is unworkable, I just don't think that it is particularly desirable for this mission.

Assuming that the solar sail and tether can be made to work the combination could also be used to clean up Earth orbitals.

Slowburn 11th April, 2012 @ 09:19 pm PDT

Re; Slowburn

"Assuming that the solar sail and tether can be made to work the combination could also be used to clean up Earth orbitals." :- If this is so we can just send up the cheapest ION drive Microsat in a block of Aerogel and stick it to the debris.

You would still have to have some form of propulsion to get a sail Micro Sat to its debris chunk, however if you use a tether and the target is in motion the odds are hitting the debris would mean it would deflect and act as a whip, severing the tether to save your Sail sat just means adding to the problem. Debris is everything imaginable, from nuts to Saturn 5 upper stages. At least Ion propulsion on your Microsat allows it to match speed & spin.

Why do we need alternate propulsion for our Sail sat ?.

Because in Earths orbit at closest approach there is a sizeable atmosphere which will make the sail act as a break and not a form of propulsion. To get anywhere with the sail it would have to be reeled in and out at every 7 minutes(1 orbit is 15 mins or more depending upon appogee). The problem is that Earths orbit is so saturated with Debris now that by the time this Sail sat has got to its debris target it would have been damaged itself.

However you could deploy sails on the debris as an aerobreak after you capture your debris and move on to the next target. There is no reason to sacrifice the Microsat itself.

It may be that the NASA team have given themselves an unrealisticly small target size for the Satelite. Microsats come in at 40Kg which is the normal entry size for a piggyback payload Satelite. It looks really good on budget hill however, however since NASA has its own launchers or in this case access to those of the US airforce the entire arguement that the Sat itself must me a certain weight/size/propulsion is incorrect as NASA can piggy back these Sats into orbit and assemble them together.

There is also a full time member of staff who does origami. Size therefore is not an issue.

L1ma 12th April, 2012 @ 10:17 am PDT

For the orbital cleaning the capture devise is reeled in to make the capture, and then reeled out lowering the captured item and giving it a velocity lower than the orbital velocity of the altitude it is at. When released the item will fall into the atmosphere and burn up at the same time releasing the weight from the bottom of the tether will lift the rest of the satellite into a higher orbit. On the other hand if you are clearing high orbitals you can put the debris on the outside and kick it father out at the same time as you lower the craft back to the debris field. For this I would aim to put the debris into the moon.

Again the solar sail craft has more delta-V than a ion engined craft of the same mass.

If you could make your Ion engine craft use collected debris as reaction mass then you would really be on to something.

Slowburn 12th April, 2012 @ 11:33 pm PDT

"If you could make your Ion engine craft use collected debris as reaction mass then you would really be on to something. "

One of the points of going to Phobos was to use the asteriod as fuel in future missions.

Still Earth orbit is not the place for Solar sails, or is no longer the place. Now if you have an external Laser powering the Sail Microsat from the say the Lunar surface we are in a different ball park.

L1ma 13th April, 2012 @ 01:42 am PDT

Re; Slowburn

Of course if we have a poweful enough laser on the Moon, we can dispense with the garbage Sat.

L1ma 13th April, 2012 @ 07:02 am PDT

re; L1ma

Granted given the amount of debris we may have to start at the top and work our way down but the solar sail craft will still be be removing garbage long after an ion rocket craft has run out of reaction mass.

Slowburn 14th April, 2012 @ 10:36 am PDT

Re; Slowburn

"Granted given the amount of debris we may have to start at the top and work our way down but the solar sail craft will still be be removing garbage long after an ion rocket craft has run out of reaction mass"

Unlikely as the Solar sail is a 5 * 5 meter or larger target for everthing from Micro metorites to our own Debris. It will have to be furled and unfurled, even in the depths of the solar system space is not 'empty' but filled with dust and without the ability to repair the micron thick sail material, eventually it will tear with impacts.

Its like with insurance, we have to launch and lose a lot of these satelites before we know exactly how often this would be, like Lloyds insuring your Clippers, we will not know how many will be lost going to India until after they first sail, then next year we can create the premiums.

So in answer, we do not know yet but we do know where this Sail Sat is going it will be exposed to great hazard for a very long period of time, without an alternate form of propulsion, with no means of repair and as of yet not even one has been made and launched.

L1ma 15th April, 2012 @ 10:27 am PDT

re; L1ma

You seem to be arguing that one should not be launched but you are correct that I should have said assuming it works.

Slowburn 15th April, 2012 @ 11:12 pm PDT

Re; Slowburn

"You seem to be arguing that one should not be launched but you are correct that I should have said assuming it works."

The arguement is not to be bought out by the hype, if I was that in the nay faction I would be recomending Gunpowder propulsion with a long taper. It is just that like the Great Eastern, one starred project seamingly ahead of its time will find itself no friends among competing techologists and it also has to have a market, as well you also have to match the Horse to the Course.

Solar sail technology has been with us since the 1950's, it has a lot of opponents who wish to use Nuclear/Chemical. It can work but only if you roll out the technology appropriately and accept it has weaknesses which must be made up for with alternate methods.

Because of its Environment is more hostlie you must take more than one sail, be able to repair the masts, be able to patch the sail itself and be able to make use of alternate energy with that sail. That means it needs robotic crew (repairbot) with sensors which will be used to inspect the craft, the means to reef sails in, and repair them. You can do amazing things with 3 D printing on a suface.

That needs extra mass but since you are not using fuel that is not a problem. However if NASA sends these devices to Mars without the aforementioned 'Extras' the lobby groups will be able to point out a failed mission as a failed technology and block development at a very cheap price.

L1ma 17th April, 2012 @ 10:28 am PDT

I did assume that a spare set of sails would be included on the Mars probe. I think a sheet of metalized plastic, or metal foil will last much better than the woven fabric of the American Flag hung on ISS.